Ciaran Martin, chief executive of Britain’s National Cyber Security Center, said Russia had targeted “millions” of devices in both countries, often seeking to hack into individual homes or small businesses or to control their routers. “Once you own the router,...

He declined to give any further information, saying that the attack was continuing, as was an investigation into who might be behind it. “A public discussion would serve as a warning to the attackers that we simply do not want...